[Hallicrafters] Restoration hints for the SX-24 and SX-25

[Mike Everette]

While working on my SX-25, I learned a few things that
may help some other people who are into these and the
SX-24's.

Recapping the 25's front end (and I suspect the 24 as
well) will be much easier if you temporarily remove
the U-shaped metal piece that runs from the front to
the rear of the chassis and is anchored to the metal
baffles which hold the goils and band switch sections.
 With this out of the way, it is a lot easier to get
to many of the caps that are connected to the coils. 
BE VERY CAREFUL of stressing the lugs during soldering
or old-part removal.  They aren't hard to break, or
pull away from the coil forms.

Don't try to remove the baffles!  They are anchored to
the chassis with sheet-metal screws, and these screws
are often brittle from age.  Using any real torque on
them, even with a hand held nut driver, may snap the
heads off (been there/done that etc).  They are close
to a #6 size with a 1/4 inch hex head, but are
actually a rather odd thread by modern standards, and
I haven't found any source of new ones.  BUT, if you
have access to any junked Motorola control heads from
tube type radios or even stuff up to and including the
MICOR series, these same type screws -- or something
much closer to the Halli screws than anything else
I've seen -- were used to hold them together.  In any
given Motorola head, you'll probably find 6 to 10 such
screws.

If you have the misfortune to break a screw head off,
use a very long and slim pair of heavy-duty needle
nose pliers to remove the screw by grabbing it on the
top side of the chassis and slowly turning it so it
will come on through to the top side.  (BE CAREFUL of
the tuning condenser!) Then replace the screw from the
bottom side, just like the original.

The best thing to do, though, is LEAVE THEM ALONE in
the first place!

The hardest caps to replace in the 24/25, in my
opinion, are the cathode and screen bypasses on the
6K8 oscillator-mixer tube socket.  If you use
conventional tools, they are probably impossible
without damaging something else, like a coil in the
neighborhood.  BUT,  if you obtain 2 special tools --
a pair of bent-nose longnose pliers, preferably 30
degree bent; and a pair of angle-tip cutting pliers --
it CAN be done.  Note, I didn't say it was easy, but
it's a lot less difficult!

The reason some 24/25 receivers may be a bit deaf on
the high band MIGHT be that the cathode resistor on
the osc-mixer 6K8 has gone high in value.  Early runs
of the 25 used a 400 ohm resistor; later runs used 250
ohms.  The one in my 25 was marked 400, but measured
750 ohms.  My 24 hasn't yet been restored but a
resistance check showed its 6K8 cathode resistor to be
high as well.  When replacing the cathode resistor in
the SX-25, I used a 270 ohm 1 watt type which is more
in line with the later production run.  Nothing else
in the 6K8 circuit appears different between runs.

I also found a Velleman 60 watt solder station useful
on this job.  It has a thin pencil iron, great for
tight spots.  Also, once the tips get hot, they can
CAREFULLY be bent to about a 30 degree angle which
proves quite handy for working in the front end area.

If you replace the caps around this socket, get both
the cathode and screen bypasses AND the cathode
resistor at the same time.  My 25 used some long leads
on one of the caps to tie some pins together and it
was not that hard once I figured out what to unsolder
and what to leave alone.  Some of that "tie wire" can
be left in place.  But above all, BE CAREFUL in this
area.

At least 80% of the resistors in my SX-25 were found
to be significantly beyond a 20% tolerance limit. 
Most were much higher than marked.  This included one
of the two large 15K carbon voltage divider resistors,
which measured almost 25K.  I replaced both of these
with 15K 5 watt wire wound types.  The other resistors
were replaced with 5% components.

I highly recommend having the power transformer,
filter choke and audio output transformers in these
receivers megger-tested and vacuum-tanked by Gary
Brown, WZ1M, to restore the insulation and help
prevent a catastrophic problem.  The line voltage in
my area rose to 135 volts one afternoon while I was
testing some work on my SX-25, and the power
transformer got really hot and started smelling like
it was about to fry.  I pulled the iron and sent it to
Gary; no more problems after it was reinstalled, and I
for sure run a Variac and voltmeter on the bench at
all times now.

My SX-25 had a severe stability issue until I
discovered another of those infamous sheet metal
screws to be broken.  This was one of the two screws
at the front of the chassis that hold the band switch
detent in place.  If you just looked hard at the band
switch knob, the radio would jump frequency like a
nervous flea.  Replacing BOTH screws with new ones
cured 97% of the problem;  the rest of it is probably
the designed-in "Halli bug" we've all encountered.

The more I get into them, I'm convinced that these
radios were never intended to be worked on beyond a
certain point.  Surely the repairmen of that era
didn't have the sort of tools we enjoy today and which
are so necessary to work on these sets.  Hallicrafters
(and others) probably hoped the owner would just buy a
replacement radio.  Hmm, not unlike today's junk....

73

Mike
WA4DLF


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